1. Field of the Invention
The present invention relates to a voltage non-linear resistor ceramic composition used for a varistor appropriate for protecting electronics from electrostatic discharge, as well as to a multilayer varistor using the same.
2. Description of the Related Art
Semiconductor devices, such as integrated circuit devices (ICs) used in electronics, may be damaged by electrostatic discharge (ESD) or the properties thereof may be deteriorated. Damage of semiconductor devices due to ESD causes significant problems in electronics, such as malfunctioning and failure. In particular, recent semiconductor devices are more easily affected by ESD, as the speed of the operation is higher. Therefore, ESD protection have been becoming more and more important in recent years for various types of electronics. As electronic parts used for this, ZnO-based varistors made of a voltage non-linear resistor are widely in use.
It is desirable for varistors used as ESD protection to have excellent properties of ESD-suppression. In addition, varistors must not be destroyed by ESD, and thus, it is necessary for them to have excellent reliability performance against ESD. Furthermore, compact varistors having a capacitance of as low as several pF which do not cause the transmission properties to deteriorate and can be used when the frequency of the circuit signal is high have been desired, because recently miniaturization of electronics is strongly desired.
However, conventional multilayer varistors do not sufficiently meet the above-described requirements in terms of their properties. ZnO-based varistors are generally categorized into two types: Pr-based ones and Bi-based ones, on the basis of the additives for gaining certain properties. Among these, Pr-based multilayer varistors are appropriate for lowering the varistor voltage. Some of them have excellent properties of ESD-suppression and the reliability performance against ESD as well as a low varistor voltage V1 mA (voltage value when current value is 1 mA) of approximately 8 V to 39 V. However, the dielectric constant thereof is generally approximately 700. Therefore, the space between electrodes is small in multilayer varistors having a low varistor voltage V1 mA of approximately 8 V to 39 V, and thus, the increase in the capacitance becomes significant, and it is difficult to lower the capacitance. Accordingly, such varistors are inappropriate for application in lines for high-frequency signals.
On the other hand, Bi-based multilayer varistors have excellent non-linearity in the voltage. In addition, some of them have a low dielectric constant of approximately 70. That is, there are multilayer varistors having a very small capacitance value of 0.5 pF to 1 pF. When the area of the electrodes is reduced together with the reduction in capacitance, the current density increases, and therefore, the properties may easily be deteriorated due to ESD when the varistor voltage is lowered to 100 V or less. Therefore, only a high varistor voltage of approximately 100 V can be implemented when designing to gain sufficient reliability performance against ESD for practical use, and thus, it is difficult to lower the varistor voltage. Therefore, the properties of ESD-suppression tend to be insufficient, and the ESD protection in the device tend to be insufficient.